Kotahi anake means: Only one.
Kotahi is the form of the word tahi/one that you typically use when you’re saying there is one of something in particular:
It’s also used in this form within larger numbers, such as when specifying that there is one ‘hundred’ or one ‘thousand’:
Numbers are often prefixed by ‘toko-‘ when referring to people or preceded by ‘e’ if referring to anything else, but kotahi (alongside anything starting with ‘tekau’) do not take ‘e’ or ‘toko-‘ (for most speakers, although there are variations in dialects):
There are some situations where you use simply ‘tahi’, such as if it is preceded by ‘ka’.
This form is also used when you are explicitly counting, focused on the process of counting:
Anake is a postposted particle (like mai) whose purpose is to indicate that the phrase it stands within is is an exhaustive list of the entities being referred to, equivalent to ‘only’ in English: